They should not rub, I have 18x10's in the rear and 18x8.5 in the front (made for Nissan 300 Turbo offset) and mine rub a tiny bit. Nissan offset is more positive than BMW. You should be good to go!!!

: I'm about to purchase some 18x8.5 Hamann HM2 wheels all around and I was wondering what tire size should I get to avoid rubbing in normal street conditions? I am not planning on aggressive driving with this set as I will use 17's for the track.